Abstract

We demonstrate a novel mechanism for prey detection in birds. Red knots (Calidris canutus), sandpipers that occur worldwide in coastal intertidal areas, are able to detect their favourite hard–shelled prey even when buried in sand beyond the reach of their bills. In operant conditioning experiments designed to find out whether the birds could tell buckets containing only wet sand from buckets containing hard objects in wet sand, we show that they detect the presence not only of deeply buried live bivalves but also of stones. The latter finding virtually excludes, under experimental conditions, prey–detection mechanisms based on vision, acoustics, smell, taste, vibrational signals emitted by prey, temperature gradients and electromagnetic fields. A failure to discriminate between food and non–food trays with dry sand indicates that pore water is involved. Based on the presence of large arrays of Herbst corpuscles (sensory organs that can measure the acceleration due to changes in pressure), the specifics of foraging technique and the characteristics of sediments on which red knots feed, we deduce that the sensory mechanism involves the perception of pressure gradients that are formed when bills probe in soft sediments in which inanimate objects block pore water flow. To our knowledge, this mechanism has not been described before. It is argued that repeated probing in soft, wet sediments allows red knots to induce a residual pressure build–up of sufficient strength to detect the pressure disturbance caused by a nearby object. The cyclic process of shaking loosely packed sand grains followed by gravitational settling into a closer packing, leads, owing to insufficient drainage of the sediment, to a locally increased pressure disturbance that is ‘pumped up’ at each shake.